Source testing cardiac pacer with source-independent rate circuitry and disabling means therefor

ABSTRACT

Externally controlled implantable cardiac pacer and heart testing apparatus. Apparatus is disclosed for use with an implantable heart-stimulating device to initiate a test mode for investigating several electro-medical areas including the implanted power source. In one test mode, induced continuous heart stimulation is provided at a stimulation rate dependent upon the level of the power source thus providing an indication of remaining life of the power source. This test mode also provides test-stimuli each of which has its energy reduced by a pre-determined amount to indicate degree of capture-margin available. One of the external controls disclosed causes another test mode in which all heart stimulation ceases and provides a physician with an opportunity to check the patient&#39;&#39;s heart and heart rate under non-stimulative conditions.

United States Patent [1 1 Goldberg Nov. 27, 1973.

[ SOURCE TESTING CARDIAC PACER WITH SOURCE-INDEPENDENT RATE CIRCUITRYAND DISABLING MEANS THEREFOR [75] Inventor: Herbert E. Goldberg,Concord,

Mass.

[73} Assignee: American Optical Corporation,

Southbridge, Mass.

22 Filed: Jan. 10, 1972 21 Appl. No.: 216,667

Primary ExaminerWilliam E. Kamm Att0rney-William C. Nealon et al.

57 ABSTRACT Externally controlled implantable cardiac pacer and hearttesting apparatus. Apparatus is disclosed for use with an implantableheart-stimulating device to initiate a test mode for investigatingseveral electro-medical areas including the implanted power source. Inone test mode, induced continuous heart stimulation is provided at astimulation rate dependent upon the level of thepower source thusproviding an indication of remaining life of the power source. This testmode also provides test-stimuli each of which has its energy reduced by,a pre-determined amount to indicate degree of capture-margin available.One of the external controls disclosed causes another test mode in whichall heart stimulation ceases and provides a physician with anopportunity to check the patients heart and heart rate undernon-stimulative conditions.

7 Claims, 5 Drawing Figures Patented NOV. 27, 1-973 2 Sheets-Sheet 1.

INTERFERENCE CONTINUOUS FIG.|

VOLT

BATTERY VOLTAGE SOURCE TESTING CARDIAC PACER WITH SOURCE-INDEPENDENTRATE CIRCUITRY AND DISABLING MEANS THEREFOR BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates in general toimplantable heart stimulating devices. More particularly, the inventionrelates to external control over such devices for providing test modeswhich allow at least determination of the extent of use of the implantedpower source and the capture-margin available, and investigation ofnonstimulative heart operation.

2. Description of Prior Art Implantable heart stimulating devices, ofboth a continuous and demand type, have been disclosed in prior art.External means for controlling operation of the implanted device hasalso been disclosed, i.e., externally magnetically operated implantedreed switch. An example of these reed switches can be seen in US. Pat.No. 3,311,111 to Bowers. However, the operation of these prior art reedswitches do not provide the test mode or modes of the present invention.

In another patent .to Bowers, US. Pat. No. 3,563,247, there is disclosedan external control for varying rate of stimulation pulses. This patentdiscloses a variation in pulse rate for the purpose of stimulating thepatient at a therapeutic rate different from the initial pacer rateprovided. Although the present invention also provides a variation inpulse rate, it is not for providing the patient with a new therapeuticstimulation rate different from the initial pacer rate. The presentinvention utilizes rate information during a temporary test mode ofoperation to provide at least an indication of remaining life of theimplanted power source. This is not disclosed in the prior art.

Certain pacers on the market today are designed to maintain a stablepulse rate regardless of battery condition. There is no apparent ratechange or other indication of forthcoming failure of the device untilseveral cells of the total number of cells have failed. Thencatastrophic failure may occur. Althoughapparent life of the pacer islengthened, actual life of the pacer is not changed at all. i

This approach to implantable pacers is desirable from a marketing pointof view, but undesirable from a life-support point of view. The patientslift depends on continued proper functioning of implanted circuitry. Thepacer may lose capture (the ability of the pacer to stimulate the heart)because of the reduced pulse energy. With this type of design, there isno readily available way of checking thebatteries at various timesthroughout the pacers life.

It is thus desirable to know how much battery life remains at varyingpoints in time after implantation. An undesirable way to determinebattery life is to utilize a surgical procedure and remove the implantedpacer to test the batteries. This is, of course, a poor approach. It isdesirable to make the determination of remaining power source life byobserving some characteristic of the pacer while it remains implanted.This characteristic of the pacer should be selected to provide anindication to the observer of the state of the power source. A solutionto this battery depletion-sensing problem is provided by utilizing anexternally manually controlled test rate mode of operation. In thenormal demand mode of operation, the pacer provides a pulse ratesubstantially independent of power source level.

The pacer may lose capture for yet another reason. The energyrequirements imposed by the heart on the pacer, to cause the heart tobeat, may increase with time. Thus, if the batteries have not beensubstantially depleted, the extra energy required by the heart may initself prevent the pacer from causing capture. The prior art does notdisclose readily available means for making this capture-margindetermination; a reduced pulse SUMMARY OF THE INVENTION The inventioncomprises externally controlled implanted test mode circuitry arrangedto cause dependence of heart stimulation rate upon level of power sourcein the test mode, but arranged to provide a heart stimulation rateindependent of level of power source in the ordinary demand mode. Testmode circuitry includes a magnetically operated reed switch, and othercircuitry to causea battery-dependent disproportionate change betweentiming-capacitor charging rate and threshold level of an implantedrelaxation oscillator. This makes the oscillator frequency and thus theheart stimulating rate controllably dependent on power source level.

An advantage of the present invention is to provide informationregarding the state of the implanted batteries without resorting to asurgical procedure to determine sarne. It is another advantage of thepresent invention to provide (during normal demand operation) arelatively constant rate selected to be the optimum rate for thatpatient. The circuitry permits maintenance of that desired rate, andprovides means for controllably investigating the battery state.

It is thus an object of the invention to provide an improved implantableheart stimulating device.

It is an additional object of the invention to provide externallycontrolled implantable test mode cicuitry to externally determinedepletion of implanted batteries.

It is a further object of the invention to provide a substantiallyconstant rate of heart stimulation on demand when not in a test mode.

Other objects and advantages of the present invention will becomeapparent to one having reasonable skill in the art after referring tothe detailed description of the appended drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graphical representationof rate characteristics of the pacer as a function of diminishingbattery voltage;

FIG. 2 is a schematic representation of an illustrative embodiment ofthe present invention;

FIG. 3 is an alternative embodiment of power source testing apparatus;

FIG. 4 is a functional representation of pacer stimulation inhibitionapparatus; and

FIG. 5 is an illustrative embodiment of part of the apparatus of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 is a schematic of anillustrative embodiment of the present invention. The schematic of FIG.2 is to be viewed in conjunction with FIG. 1 of U.S. Pat. No. 3,528,428,the combination of which provides an operative embodiment. The subjectmatter of this patent is incorporated herein by reference. The followingcomponents of FIG. 2 of the instant application are identical tocomponents of FIG. 1 of this patent: transistors T7, T8 and T9;resistors 61, 63 and 59; capacitor 65; electrodes E1 and E2; andconductors 9 and 11. Interconnection of these components is described inthis patent. Other components in FIG. 2 of the instant application maybe equivalent or equal to components shown in FIG. 1 of this patent, butare given different refer ence numerals.

With reference to FIG. 2, showing component interconnection, the anodeof diode 102 is connected to the junction of resistor 59, and capacitor65. The cathode of diode 102 is connected to the junction of resistors101 and 103. The other end of resistor 101 is connected to one end of anexternally operated implantable magnetic reed switch 100. The other endof reed switch 100, the fixed end, goes to conductor 150.

The other end of resistor 103 is connected to a junction comprised of acathode of diode 104, the anode of diode 105, and resistors 108 and 109.Diodes 104 and resistor 108 are in parallel connection. Diodes 105, 106and 107 are in a series string and are in a parallel connection withresistor 109. The junction of resistor 109 and the cathode of diode 107are connected to one end of parallel combination of capacitor 111 andresistor 110, the other end of which parallel combination is connectedto conductor 9. The junction of resistors 109 and 110 (junction .l) isconnected to the base of transistor T7. The potential at this junctionis the threshold level for the timing circuitry, and will be discussedbelow in detail.

Now, interrelating the circuitry of the instant application with U.S.Pat. No. 3,528,428, conductor 150 is connected to the emitter oftransistor T1 in FIG. 1 of this patent. Conductor 140 is connected tothe junction of resistors 33 and 35 in this patent. Conductor 130 isconnected to junction of potentiometers 35 and 37 in this patent.Conductor 120 is connected to the junction of capacitor 57 and theemitter of T6 in this patent. In this illustrative embodiment of thepresent invention, switch S of FIG. 1 of this patent is omitted (i.e.,consider switch S to be held open at all times).

In operation, first consider the situation with switch 100 open as isshown. Current from the series string of batteries flows throughresistor 59 and charges up capacitor 65 which holds the voltage as longas transistor T9 is not turned on. Current from the series string ofbatteries also flows through the parallel combination of diode 104 andresistor 108; the individual currents of that parallel branch combine toflow almost exclusively through resistor 109.

Diodes 105, 106 and 107 although in parallel with resistor 109, conductnegligible current in this static situation. Resistor values ofresistors 108, 109 and 110, which form a voltage divider, are chosen tokeep voltage across diodes 105, 106 and 107 below their combined forwardvoltage drop. This voltage is sufficiently low to prevent the diodesfrom conducting significantly in a forward direction. Diodes 105, 106and 107 conduct significant current only during initial turn on of thecircuitry when the batteries are initially connected. The diodes areused to counteract adverse transients during turn on. However, after asteady state situation is established, these diodes are functionally outof the circuit. They can be ignored without sacrificing anyunderstanding of the operation of the instant invention.

With switch open, as shown, normal demand mode of operation ispermitted. If the heart demands a stimulating impulse, transistors T7and T8 cause T9 to conduct, causing capacitor 65 to discharge throughelectrodes E1, E2, and the heart (not shown). In order to cause thisstimulation, the potential on the emitter of T7 must exceed thepotential on its base. This is described in the U.S. Pat. No. 3,528,428in detail.

Capacitor 111 is a component not found in this patent. It provides afunction not heretofor described. When T8 conducts, some of the currentfor the collector of T8 comes from the charged up capacitor 111. AfterT8 stops conducting, capacitor 111 recharges to its former state throughresistors 108 and 109. Capacitor 111 is chosen so that it does notrecharge to its previous static voltage value in a time equal to or lessthan the time between pulses of an ordinary heart rate. Thus, when asecond stimulation pulse is demanded after the first stimulation pulse,capacitor 111 has not charged up to its previous static state. Thus,voltage at junction J is slightly lower than it previously was.Threshold J is overcome by voltage at the emitter of transistor T7earlier than it was for the first stimulation pulse. Voltage oncapacitor 111 charges and discharges in this manner and thus rippleswhile successive stimulation pulses are being generated. Successivestimulation pulses are separated from each other by respective timeintervals that are each less than that time interval between the lastnatural heartbeat and the first stimulation pulse. This is known in theart as rate hysteresis and is not the subject matter of the presentinvention but is presented for purposes of completeness.

Bearing in mind that switch 100 is open, consider the function of diode104. Diode 104 is a compensating diode which compensates for thenon-linear baseemitter voltage drop of transistor T7. When the batteriesare fully charged and operating, component values for resistors 108, 109and 110 are selected so that current drawn from these batteries isshared by resistor 108 and diode 104. The diode current could beapproximately 0.5 microamps; it is sufficient current through diode 104to provide compensation for the baseemitter junction of transistor T7when the batteries start to diminish in voltage. (The base-emitterjunction is essentially a diode also).

For example, consider one of the batteries B1-B5 to fail. The currentthrough diode 104 is diminished somewhat in accordance with constraintsimposed by voltage divider action of resistor 108 in combination withresistors 109 and 110. (Recall that diodes 105, 106 and 107 arefunctionally out of the circuit.) The decrease in supply voltage is feltby timing capacitor 57 (shown in U.S. Pat. No. 3,528,428) which chargestowards a lower voltage, and is also felt at junction .1. (In order toavoid unnecessary repetition, certain components having the followingreference designations are shown in U.S. Pat. No. 3,528,428 and are notshown in FIG. 2 of the present application: transistors T1, T6;resistors 19, 59; capacitor 57.) It is the compensating effect of diode104 which allows junction J to decrease in voltage in a manner (withrespect to decrease of charging rate of capacitor 57) so as to hold thedemand rate approximately constant. Thus, decreased overall supplyvoltage will not substantially change the stimulation rate of the pacer.This is depicted in FIG. 1. The demand mode rate is shown to beapproximately flat from maximum battery voltage to approximately 4volts. In a particular case this corresponds to the failure of twocells.

By contrast, if diode 104 did not exist (was open) junction J woulddecrease in proportion to the decrease of supply voltage in a linearfashion because of pure resistive voltage divider action. But, thebase-emitter voltage drop of transistor T7 is a relatively fixed amountwhich is added to the linearly decreased voltage at threshold J toarrive at that voltage to which capacitor 57 must charge prior togenerating a stimulating impulse to the heart. Thus, if diode 104 wereopen or were missing, the rate versus battery voltage characteristic ofFIG. 1 would not be relatively flat.

By comparison consider a first magnet mode or a first test mode. Switch100 is a magnetically operable reed switch and is closed in response toeffects of an external magnet (not shown). Current flows through diode104, resistor 103, resistor 101 and resistor 19 to ground. Currentthrough diode 104 when switch 100 is closed is approximately times theamount of current through diode 104 when switch 100 is open. Othercurrent differentials can be used besides the factor of approximately10. The increased current through diode 104 causes its voltage drop tosubstantially increase. This, in turn, causes the potential at thresholdJ to be decreased from total battery by an equal amount. The decrease inpotential at junction J accounts for part of the marked increase inheart stimulation rate from the demand mode curve to the magnet modecurve as depicted in FIG. 1. (The reason for the other part of theincrease is described below.) The decreased threshold voltage at Jenables capacitor 57 to charge to that decreased threshold level morerapidly providing the increased rate.

Now, consider a failure of one or two batteries with switch 100remaining closed. The total battery voltage supplied is reduced by about20-40 percent. Current flow through diode 104 does decrease but due tothe nonlinearity of diode 104, its forward voltage drop remainsapproximately constant. Thus, voltage change at threshold J is a greaterpercentage decrease than total battery voltage percentage decrease. Inother words, threshold J voltage decreases proportionately faster thanthe total battery voltage decreases. At the same time, the voltagetowards which capacitor 57 is charging decreases linearly with totalbattery voltage. As capacitor 57 charges under these conditions T7 isturned on earlier. As shown in FIG. 1, the rate of stimulation increaseswith decreasing total battery voltage. One can measure stimulation ratein the magnet mode and (from curves similar to those in FIG. 1)determine to what extent the batteries have become depleted and/or havefailed.

Clsoure of switch 100 also causes other functionings. For example, thepacer is caused to be in a continuous stimulation test mode. This isaccomplished by causing transistor T1 to cut off (and not detect anyheartbeats) because of a voltage impressed at the emitter of T1 throughresistor 59, diode 102, and resistor 101. Con tinuous stimulation isnecessary when testing battery level; otherwise, if the patients heartwere functioning normally and no pacer-generated stimuli appear, then nomeasurements may be taken.

However, the aforementioned disabling of transistor T1 in the magnetmode prevents transistor T6 from conducting in response to a heartbeat(either natural or stimulated). The collector to emitter drop oftransistor T6 is approximately 0.1 volts. The double base-emitterjunction drop of transistors T7 and T8 is approximately 0.5 volts. Thus,in the mode where switch is closed, capacitor 57 is caused to rechargefrom a higher voltage pedestal (0.5 volts vs. 0.1 volts). This pedestaleffect in itself will permit capacitor 57 to charge to trigger voltagein a shorter time then when switch 100 is open. This accounts for theother part of the rate increase between the demand mode curve and themagnet mode curve of FIG. 1.

In addition to the above two functionings, closure of switch 100 causesa reduced stimulating pulse amplitude output. Switch 100 has the effectof creating a voltage divider between resistor 59, resistor 101 andresistor 19. The effect of this divider is to reduce the amount ofvoltage to which capacitor 65 charges. This provides a reduced outputpulse height and output pulse energy. This test mode allows one todetermine the ability of the pacer to provide capture for a particularheart at a reduced pulsed energy. Initially, when electrodes areimplanted in the heart, one or 2 milliamps may normally be required tocause capture for that heart at that time. After a period of time, theelectrodes may move slightly, and/or scar tissue may develop in theheart at the point of stimulation. This may result in a larger amount ofcurrent being required for capture. For example, if l or 2 milliamps wasrequired for capture and 15 milliamps was supplied initially, an initialmargin of safety of about 7:1 is achieved. But if 12 milliamps is laterrequired for capture, with 15 milliamps still being supplied, the marginof safety is substantially reduced.

The pulse height reduction feature of the present invention allows adoctor to check the margin of safety of the pacer in his office. This isa good environment in which to make such a check. The reduced pulseheight of about 30 percent (in a particular design) reduces thestimulation energy to the heart. If this reduced stimulation pulse failsto cause the heart to beat, (to cause capture) the doctor knows that themargin of safety is very slim and appropriate medical action can beimmediately taken. Of course, pulse energy can be reduced by decreasingthe pulse width and by other ways. Reduction of pulse energy is acriteria, and reduction of pulse amplitude is a way to accomplish this.Charge is another criteria.

Therefore, closure of switch 100 in this first magnet mode causes (1)reduced output pulse amplitude, (2) rate of stimulation pulses to beincreased and to increase as a function of diminishing battery voltage,and (3) the pacer to stimulate continuously, These three functions occursimultaneously.

Considering FIG. 3, there is presented an alternative embodiment forperforming functions (2) and (3). Reed switch 300 has normally closedposition NC and normally open position NO. When an external magnet (notshown) is brought in proximity to the switch, contact is made withterminal NO. Note that the continuous stimulation criteria is achievedby applying voltage to conductor 150 when in this magnet mode.

The anode of diode 303 is connected to terminal NC, the cathode beingconnected to the junction of cathode of diode 302 and one end ofresistor 304. The anode of diode 302 is connected to conductor 150 andthe cathode of diode 301. The anode of diode 301 is connected toterminal NO. The other end of resistor 304 is connected to junction Jand one end of resistor 305. The other end of resistor 305 is connectedto the most negative battery terminal, the most positive batteryterminal being connected to reed switch 300. This circuitry can replaceresistors 108, 109, 110, 101, 103 and diodes 104, 102 shown in FIG. 2.

In operation, with switch 300 in the position shown, diode 303compensates in a manner equivalent to diode 104. As total batteryvoltage decreases, the rate stays relatively flat. But, if switch 300 ismade to contact terminal NO, then the voltage at junction J decreases byvirtue of an increased diode drop, thereby increasing the stimulationrate. If total battery voltage diminishes in this magnet mode thedecrease is felt more strongly at junction J than it is felt by timingcapacitor 57. (As before, capacitor 57 appears in U.S. Pat. No.3,528,428 and does not appear in FIG. 3 of the present application.)This difference is due to the relatively constant forward voltage dropmaintained across both diodes 301 and 302 which tends to decrease thevoltage at junction .I by an amount almost equal to the battery voltagedecrease. By comparison the voltage towards which capacitor 57 chargesis reduced in a manner akin to that achieved by resistive voltagedivider action. As before, the net effect is for the measurablestimulation rate to increase with decreasing total battery voltage.

It should be understood that other circuitry can be provided whereby therate tends to decrease rather than increase with decreasing batteryvoltage. For example, this can be accomplished in general byinterchanging the roles of diodes and resistors in FIG. 3 along withmaking other suitable modifications. Thus, when the total batteryvoltage decreases, junction J will not decrease by an almost equalvoltage, but will tend to remain almost constant. This will provide adecreasing stimulation rate with decreasing battery voltage. A change inrate is required to determine battery depletion; the direction of changedoes not provide any additional information and is not all thatcritical.

Considering FIG. 4, pacer 203 is intended to contain all of thatcircuitry disclosed in FIG. 2 (except for switch 100) in conjunctionwith its operative connection to FIG. 1 of the U.S. Pat. No. 3,528,428.As shown, switch 100 and pacer 203 are contained within a suitableencapsulation 200 that is compatable with the environment of human body.The pacer is shown in operative connection with heart 202, all of whichis contained within patients body 201.

External to body 201 is shown a variable-rate, pulsed, magnetic fieldgenerator. This is shown to be in magnetic field communication withimplanted switch 100. In an illustrative embodiment, this generatorcomprises an electromagnet connected to an external electrical sourcethrough a switching device that can be manually controlled to switch ata variable rate. Typically, the switching device is a variable frequencyoscillator and a transistor switch controlled by the oscillator. This isdepicted in FIG. 5. Battery 500 is shown as the power source. Inductor510 provides the magnetic field. Diode 550 provide a discharge path forinductor 510. Transistor 520 is a switch controlled by oscillator 530which is powered by source 500 also. Potentiometer 540 allows theoscillator frequency to be manually controlled over a wide range.Certainly, other arrangements can be used.

In operation, when the oscillator frequency is set to provide betweenabout two and five pulses per second, reed switch makes and breaks atthis rate. Transistor T1 is thus caused to vary from dynamic operationto cut off causing its collector voltage to approach battery potentialat this rate. This pulsation is conducted through the remainingcircuitry as if each pulse were a detected heartbeat. Each virtualheartbeat causes transistor T6 to conduct thereby discharging capacitor57 long before it could achieve sufficient potential to cause astimulation pulse to be generated. Thus, a nonstimulative mode isprovided during which a physician may conduct a medical examination ofthe heart unfettered by continuous or sporadic pacer stimulation.

Alternatively, the frequency of oscillator 530 can be increased to morethan about 15 pulses per second and can simulate detected interference.In this situation, capacitor 49 does not charge or dischargesufficiently between each pulse to cause turn-on of transistor T6. Thus,stimulation to the heart is continuously provided via theinterference-mode, without the necessity of using a high-powered R-Fgenerator to cause the interference.

A doctor may purposely want to cause the pacer to go into aninterference-continuous mode for several reasons. One reason is to causethe pacer to generate stimulating impulses for measurement purposes whenthe pacer is inhibited by normal heart action. The rate at which thepacer will supply stimuli in this mode is close but not equal to thepreset demand rate. These two rates differ because capacitor 57discharges more through transistor T6 (one saturated transistor) than itdoes through T7 and T8 (two saturated transistors). The former dischargeprovides a slower rate. Thus, even for a new patient, (refer to theinterferencecontinuous mode curve of FIG. 1 with no prior medicalhistory available, a doctor can determine the preset demand rate bycausing an interference-mode, causing T7 and T8 to conduct, measuringthe stimuli rate, and subtracting a known number of beats per minutefrom that measured rate. This provides the doctor with demand rate forthat implanted pacer. The doctor can then determine how depleted theimplanted batteries may be by measuring the rate in the first magnetmode. This presupposes knowledge of the magnet mode rate at implantationand this data would be available for the medical profession. A curvesimilar to FIG. 1, but including guantitative voltage and rateinformation, would be supplied as well as other related curves. Thistest method and apparatus provides a determination of implanted batterycondition without requiring any substantial medical history of thepatient even under the condition of normal inhibition.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

I claim:

1. In an improved implantable electronic heart pacer having aself-contained power source, said pacer including terminal means forconnection to the heart of a patient, pulse generator means connected tosaid source for supplying heart stimulating impulses at predeterminedrates on said terminal means, the improvement comprising:

means for maintaining a heart stimulation rate substantially independentof remaining life of said source;

test mode means including manual means for disabling said maintainingmeans and for making said stimulation rate dependent upon and indicativeof remaining life of said source.

2. The improvement of claim 1 and wherein said testmode means furtherincludes means for providing said impulse rate independent of thebeating action of said heart.

3. The improvement of claim 1 and wherein said pacer is a demand pacerwhich includes means for detecting the beating action of said patientsheart, and means responsive to said detecting means for enabling saidpulse generator means to supply said impulses only in the absence ofdetected heartbeats.

4. The improvement of claim 1 including means for increasing saidimpulse rate with depletion of said source.

5. The improvement of claim 1 including means for decreasing saidimpulse rate with depletion of said source.

6. The improvement of claim 1 including means for controllablycontinuously supplying said test mode impulses.

7. A method for determining the remaining useful life of anunrechargeable power source used for powering an implantable electronicpacer which is used for providing stimulation to the heart of a patient,said method comprising the steps of:

a. manually causing the otherwise batteryindependent rate of stimulationto be dependent upon the level of said power source;

b. measuring said dependent rate of stimulation;

c. comparing the dependent rate of stimulation with a known rate versussource-depletion characteristie; and,

d. Determining remaining useful life of said power source from saidcomparison.

1. In an improved implantable electronic heart pacer having aself-contained power source, said pacer including terminal means forconnection to the heart of a patient, pulse generator means connected tosaid source for supplying heart stimulating impulses at pre-determinedrates on said terminal means, the improvement comprising: means formaintaining a heart stimulation rate substantially independent ofremaining life of said source; test mode means including manual meansfor disabling said maintaining means and for making said stimulationrate dependent upon and indicative of remaining life of said source. 2.The improvement of claim 1 and wherein said test-mode means furtherincludes means for providing said impulse rate independent of thebeating action of said heart.
 3. The improvement of claim 1 and whereinsaid pacer is a demand pacer which includes means for detecting thebeating action of said patient''s heart, and means responsive to saiddetecting means for enabling said pulse generator means to supply saidimpulses only in the absence of detected heartbeats.
 4. The improvementof claim 1 including means for increasing said impulse rate withdepletion of said source.
 5. The improvement of claim 1 including meansfor decreasing said impulse rate with depletion of said source.
 6. Theimprovement of claim 1 including means for controllably continuouslysupplying said test mode impulses.
 7. A method for determining theremaining useful life of an unrechargeable power source used forpowering an implantable electronic pacer which is used for providingstimulation to the heart of a patient, said method comprising the stepsof: a. manually causing the otherwise battery-independent rate ofstimulation to be dependent upon the level of said power source; b.measuring said dependent rate of stimulation; c. comparing the dependentrate of stimulation with a known rate versus source-depletioncharacteristic; and, d. Determining remaining useful life of said powersource from said comparison.